When is violence acceptable?

[Quantum Windbag]

When in politics is violence acceptable? It's become clear that there are some on the right and the left calling for violent revolution. It's also become clear that there are people on both sides of the aisle who have no problem justifying/excusing it when their guys call violence.

But when is actually acceptable?

From my limited understanding of history, whenever it accomplishes the goals of the person committing violence.

On a more personal level, whenever the state tries to take away freedom form anyone.